A clutch assembly of a motor vehicle is arranged to transmit drive power of the motor vehicle to a wheel of an axle. The clutch assembly includes a clutch device that can be engaged and disengaged, and that can transmit drive power from a first drive member to a second drive member. The first drive member is on the drive side of the clutch device. The second drive member is on the output side of the clutch device. An oil-delivering device is provided to oil the clutch. The oil-delivering device delivers oil of an oil circuit serving the clutch device depending on an operating state. The clutch device is mounted in a housing which forms a clutch space for accommodating the clutch device.
The clutch assembly may be part of a clutch-controlled equalisation unit, by which wheels of an axis can be integrated into the flow of drive power, or decoupled from the same, in order to be able to operate a vehicle in all-wheel drive or two-axle drive, or in two-wheel drive or single-axle drive, as desired.
A clutch assembly of this type is known from EP 2 116 411 A1. In a clutch-controlled equalisation unit, a clutch assembly is provided with an activatable multiple disc clutch for a side shaft, which forms a drive member on the output side of the clutch device. Drive power is directed via the multiple disc clutch to the connected drive wheel of a motor vehicle as required. The outer disc carrier of the multiple disc clutch, which can be coupled to the drive-side shaft or to the output-side drive member, delivers lubricating and cooling oil into an oil pan provided in a housing of the clutch assembly, from which the oil flows back into a sump formed by the clutch space, is picked up again by the clutch, and is fed back to the circuit. An inner oil circuit is thereby formed in relation to a disc pack of a side shaft clutch.
Because of this inner oil circuit, a large proportion of oil used for cooling and lubrication remains for a long time in the described inner oil circuit without the oil volume being sufficiently mixed with, or replaced with, “fresh” oil. The oil volume that has only just absorbed heat between the clutch discs is fed directly back to the discs. The clutch temperature therefore increases over-proportionately to the cooling effect that would be achievable with the oil quantity present in the equalisation unit. The over-proportionate heating of the clutches is disadvantageous in terms of the wear behaviour and thus the service life, as well as in terms of the response and control behaviour.
A further disadvantage of the configuration disclosed in EP 2 116 411 A1 is that the clutch packs of the side shaft clutches are in the oil continuously in order to pick it up and deliver it to the oil collection pocket. The associated splash losses increase the power loss of the drive train.
With regard to the undesirable power loss, it is also the case, even in the case described in EP 2 116 411 A1, that, where the secondary part of the drive train is decoupled from the drive wheels and the side shaft clutches are completely open, the secondary drive wheels roll on the road in driving mode and drag a clutch disc carrier and the discs connected in a rotationally fixed manner thereto. The discs, however, are in constant contact with the oil, because they dip therein. Not only is the region of the extremely narrow air gap (gap width approximately 0.1 mm) between the inner and outer discs, which is situated directly in the oil, filled with oil, but the oil is also conveyed into the interspace, not in the oil, between the inner and outer discs of the multiple disc clutch by the rotating discs. The liquid friction or hydrodynamic friction produced as a result in turn transmits drag torque acting on the inner discs from the secondary drive wheels to the outer discs, thereby driving the outer discs. However, as soon as the outer discs begin to rotate, they in turn convey oil into the above-described inner oil circuit, and thus increase the oil volume conveyed, and therefore likewise again increase the friction and power loss. The undesirable effect intensifies by itself to a certain extent.
Experiments have shown that, because of this effect, the secondary drive train is not immobilised even when decoupled from the primary part of the drive train, in contrast to the assumptions made in EP 2 116 411 A1. The negative consequences of this effect can be reduced only slightly by the low-friction design of the surfaces of the inner discs proposed in EP 2 116 411 A1. Moreover, low-friction surfaces of the inner discs result in a much poorer response and control behaviour of the clutch pack. This disadvantage outweighs the power loss advantages that can be achieved with this measure.
Given this background, a clutch assembly of the type mentioned in the introduction, is desirable to avoid the above-described disadvantages. E.g., For example, it is desirable to optimise the oiling concept described in EP 2 116 411 A1 with the aim of minimising power loss when a secondary axle drive wheel is decoupled from the flow of drive power by means of the clutch device.